Great quality (but budget priced) 2D and 3D artwork you can use in your own games.
Game Development Crash Course
19 lessons and an hour of video get you started fast and free with Corona SDK game dev!
Super Sprite Bundle
Royalty-free graphics for 2D games - animated characters, tile sets, etc.
OGT Level Manager
Lock and unlock levels easily with OGT Level Manager for Corona SDK.
App Vault
Biz app developers - these help you create stunning apps in half the time while saving you tons of money you would have otherwise spent hiring designers.